1. Field of the Invention
This invention relates to the field of electrogalvanic batteries. In particular, it relates to primary batteries having cathodes containing lead chloride and which are activated by immersion in seawater.
2. Description of the Prior Art
Primary, i.e. single discharge, batteries using lead chloride cathodes with metallic anodes such as magnesium or zinc are known in the battery art. In one application, such batteries provide a convenient source of power for various sorts of electronic apparatus used for monitoring underwater sound. A particularly favorable feature of these batteries is that they will operate using seawater as the electrolyte. The batteries are activated simply by immersion in a conductive aqueous solution such as seawater thereby eliminating the need for any electrolyte storage and dispensing devices.
It has been usual practice to have the seawater battery fully connected to its load at the time of immersion. In normal battery design, it may require one or more minutes for the battery to reach operating voltage. This is particularly true when the battery, the electrolyte or both are at a low temperature. Often the discharge is planned to last for several hours so that a delay of a minute or so between immersion and the attainment of working voltage is of little importance. However, in a certain class of device, it is important that the battery reach a useful voltage under a considerable load within a fraction of a minute and under adverse temperature conditions. Lead chloride seawater batteries of prior art design do not activate rapidly at low temperatures.
Seawater batteries are known using anodes of sheet magnesium, sheet zinc and sheet aluminum. It is also known that alloys of these materials offer improved characteristics compared to pure metals. The magnesium group offers the highest cell voltages.
A cathode for a seawater battery has been disclosed in the prior art wherein lead powder is mixed with lead chloride and a small quantity of graphite and the combination hot pressed onto a copper mesh grid or conductor. The purpose of the lead powder addition is to act as a binder to hold the lead chloride in place on the copper screen. Cathodes made in this manner have superior voltage rise time characteristics after a storage period when compared to cathodes made with plastic binders. The voltage rise time improvement is believed to be due to the protection during the storage period of the copper mesh by lead particles in the cathode mix. The recommended ratio of lead powder to lead chloride in this prior art cathode is 30 parts by weight lead powder to 70 parts by weight of lead chloride. Mixes having a lower ratio of lead powder to lead chloride down to 10 parts lead to 90 parts lead chloride are also disclosed in the prior art.
Polyfluoroethylene (PFE) has been added to many types of galvanic electrodes. It was first used in gas electrodes to give the electrode surface anti-wetting properties. Subsequently, it has also been added as a binder to electrodes used in the fully submerged state. The material has been shown to be a better binder for immersion electrodes than the older plastics such as the polystyrenes, polyolefins, methacrylates, etc. Electrodes containing polyfluoroethylenes can be discharged at higher rates than electrodes with the other plastic binders mentioned. The most commonly used polyfluoroethylene is polytetrafluoroethylene (PTFE), however, it has been shown that other polymerized halogenated ethylenes containing fluorine have similar properties. Therefore, the term polyfluoroethylene is used in this specification in a generic sense to include all such compounds.